Integrated circuits are now used in almost every type of electronic product ranging from toys to massive computers. These integrated circuits are all generally made by a photolithographic process, which involves manufacturing a template containing patterns of the electrical circuit as transparent and opaque areas. The patterned template is referred to as a “reticle” or “mask”.
A radiation source, such as a light, is used to copy or “pattern” multiple images of the mask onto a photosensitive material, such as a photoresist, on the surface of a silicon wafer. Once features are patterned on the photoresist, further processing is performed to form various structures on the silicon wafer. The completed wafer is then cut (or “diced”) to form the individual integrated circuits.
Engineers typically use computer aided design (“CAD”) to create a schematic design of the mask. One technique, Levenson Phase-Shifting, also known as Alternating Aperture Phase-Shifting, is used to create small features on integrated circuits. Such small features are generated by a pair of areas in the mask called shifters.
Shifters change the phase of the light passing through them. Two shifters can be used to shine light on the same region of a photoresist. When the light passing through one of the shifters is out of phase with the light passing through the other shifter, a feature is created on the photoresist that is narrower than the distance between the two shifters. By reducing the distance between the two shifters, very small features can be created on the photoresist. The width of the feature can be considerably less than could be produced by the same optical system without phase shifting.
However, the use of phase shift masking in dense designs results in a large increase in the complexity of mask layout. One problem, when laying out phase shift areas on dense designs, is phase conflicts. Phase conflicts are due to poor phase assignment and typically occur at line-end and T-junction structures.
One type of phase conflict occurs when two phase shift regions having the same phase are laid out to create a feature. If the phase shift regions have the same phase, they will not create the desired feature. Thus, it is necessary to correct the phase shift regions so that they are no longer in the same phase.
However, correcting phase shift problems in one area of a complex mask layout may produce phase problems in other areas of the mask layout. As designs become more complex, the time and effort involved in correcting phase shift problems increase dramatically.
Conventional design systems try to reassign phases of individual pairs to resolve conflicts at the end of the design process, when all the phase conflicts are apparent. Sometimes, however, a phase shift conflict cannot be resolved. Discovery of such irresolvable phase shift conflicts requires the design engineers to start over at the early CAD process stage. The design and conflict resolution processes are then repeated until final design layouts without phase-shift conflicts are produced.
Unfortunately, this procedure greatly increases the number of days it takes to begin producing integrated circuits. In the modern marketplace, where advancements occur daily, such delays can cause significant loss of market share and revenue.
To solve this problem, chip manufacturing facilities have imposed design rules to restrict engineers from creating patterns which result in phase shift conflicts. However this solution overly restricts engineers and excessively limits designs. What is needed is an easier, faster, and more efficient way to resolve phase shift conflicts.
Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.